A conventional electromagnetic actuator for opening and closing a valve of an internal combustion engine generally includes "open" and "close" electromagnets which, when energized, produce an electromagnetic force on an armature. The armature is biased by a pair of identical springs arranged in parallel. The armature is coupled with a gas exchange valve of t he engine. The armature rests approximately half-way between the open and close electromagnets when the springs are in equilibrium. When the armature is held by a magnetic force in either the closed or opened position (at rest against the open or close electromagnet), potential energy is stored by the springs. If the magnetic force is shut off with the armature in the opened position, the spring's potential energy will be converted to kinetic energy of the moving mass and cause the armature to move towards the close electromagnet. If friction is sufficiently low, the armature can then be caught in the closed position by applying current to the close electromagnet.
In order for the gas exchange valve to rest firmly against the valve seat and simultaneously allow the armature to contact the "close" electromagnetic, tolerances would need to be near zero on the individual components. This is not feasible and would still not account for differential thermal expansion and wear. Simultaneously seating of the gas exchange valve and armature requires an adjustable element to be included between the armature and gas exchange valve. One typical type of adjustable element used conventionally on most modern engines with cam driven valves is a hydraulic tappet, also known as a hydraulic lash adjuster. For proper function, the hydraulic lash adjuster should have a very high stiffness when acted on by a transient load, such as during the motion of the gas exchange valve, but must be compliant during the dwell time of the gas exchange valve on the valve seat. The function described above can be achieved by using a piston-cylinder assembly with a check valve and leakage orifice with the operating fluid typically being the engine motor oil. The peak static force which the lashed adjuster can exert in order to expand is determined by the piston area and the engine oil pressure, and is typically very small. The force required to retract the lash adjuster is very large, and is determined by the leakage orifice area which is typically only the annular clearance between the piston and the cylinder diameter.
Prior to the start of the system, oil pressure is zero and it can be assumed that the lash adjuster has leaked down to its minimum length. When the engine is started, the gas exchange valve begins to move as necessary for engine operation and oil pressure begins to build up. Valve operation starts prior to oil pressure being available to the lash adjuster and, until oil pressure is available, a gap will exist between the armature and the gas exchange valve when the valve is in the close position (referred to typically as valve lash). Once oil pressure is available, the cylinder in the lash adjuster will fill with oil and begin to expand the piston until zero lash is achieved.
It has been found that, when used with an electromagnetic actruator, the conventional hydraulic lash adjuster can become too long due to unwanted harmonic motion of the gas exchange valve. For example, if the armature hits the "open" electromagnet at a higher than desired impact velocity, the kinetic energy of the gas exchange valve will keep it moving in the open direction after the armature has stopped, releasing any load on the hydraulic lash adjuster (valve toss). If oil pressure is applied to the hydraulic lash adjuster when the adjuster is unloaded, it will undesirably extended in length. Accordingly, a need exists to prevent undesired filling of the lash adjuster so as to control when the piston of the lash adjuster is extended.
During operation of the actuator, another concern is lubrication of the moving parts and cooling of the actuator. Typically, a shaft or guide is associated with the armature and moves within bushings. If the shaft is not lubricated, high wear and high friction in the bushings may result during operation of the actuator. It is difficult to provide enough lubrication to the shaft since oil is available at the outside of the bushings, but lubrication is needed on the inside of the bushings between the shaft and the bushings. Accordingly, here is a need to provide metered lubrication of the shaft at the in side of the bushings.
In addition, the power required to operate an electromagnetic actuator produces heat. This heat affects the performance and durability of the actuator. Currently, the conventional methods used to remove heat from the actuator is conduction from the actuator to the engine block and by varying the amounts of convection to the surrounding environment. Accordingly, there is a need to provide oil cooling of an electromagnetic actuator to reduce the chance of the actuator temperature building to undesirable levels during high duty cycle periods.